U.S. patent number 5,251,218 [Application Number 07/739,593] was granted by the patent office on 1993-10-05 for efficient digital frequency division multiplexed signal receiver.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Don C. Devendorf, Kikuo Ichiroku, Edwin A. Kelley, Wade J. Stone.
United States Patent |
5,251,218 |
Stone , et al. |
October 5, 1993 |
Efficient digital frequency division multiplexed signal
receiver
Abstract
A digital radio frequency (RF) or intermediate frequency (IF)
receiver for frequency division multiplexed (FDM) signals contained
in a predetermined FDM band including an RF amplifier, an RF
bandpass anti-alias filter, and an analog-to-digital (A/D)
converter. The sample frequency F.sub.s of the A/D converter is
lower than the lowest frequency in the predetermined FDM band and
is selected to meet certain specified conditions based on the
passband and stop band edges of the anti-alias filter so that the
output of the A/D converter contains a non-distorted aliased
frequency down converted digital version of the predetermined FDM
band which is located between 0 Hz and one-half the sampling
frequency. A digital complex mixer responsive to the digital output
of the A/D converter translates the spectrum of the sampled digital
received signal to center the desired FDM channel at zero frequency
(DC). Digital low pass filtering isolates the desired channel
centered at DC, and a digital complex mixer can be used to
translate the isolated selected channel to a predetermined IF
frequency. The in-phase portion of the digital IF centered selected
channel or the DC centered complex envelope selected channel can
then be provided to appropriate demodulation or decoder
networks.
Inventors: |
Stone; Wade J. (Topanga,
CA), Ichiroku; Kikuo (Santa Monica, CA), Kelley; Edwin
A. (Los Angeles, CA), Devendorf; Don C. (Carlsbad,
CA) |
Assignee: |
Hughes Aircraft Company (Los
Angeles, CA)
|
Family
ID: |
26968215 |
Appl.
No.: |
07/739,593 |
Filed: |
July 31, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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293894 |
Jan 5, 1989 |
5058107 |
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Current U.S.
Class: |
370/343; 370/481;
370/484; 370/497; 455/189.1; 455/324 |
Current CPC
Class: |
H04J
1/05 (20130101) |
Current International
Class: |
H04J
1/00 (20060101); H04J 1/05 (20060101); H04J
001/00 () |
Field of
Search: |
;370/50,69.1,120,123,70,72,62 ;375/82 ;333/166
;455/323,324,179.1,180.1,182.1,182.2,182.3,188.1,189.1,190.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Multirate Digital Signal Processing", Crochiere and Rabiner,
Prentice-Hall, Inc., Englewood Cliffs, N.J. 07632, 1983, pp.
42-47..
|
Primary Examiner: Safourek; Benedict V.
Assistant Examiner: Hsu; Alpus H.
Attorney, Agent or Firm: Grunebach; Georgann S. Gudmestad;
Terje Denson-Low; W. K.
Parent Case Text
This is a continuation-in-part of co-pending U.S. application Ser.
No. 07/293,894, filed Jan. 5, 1989, now U.S. Pat. No. 5,058,107.
Claims
What is claimed is:
1. A frequency division multiplex receiver for isolating a
frequency division multiplex channel contained within a
predetermined frequency division multiplex (FDM) band,
comprising:
an RF amplifier;
an RF bandpass anti-alias filter having a passband between a lower
passband frequency f.sub.pl and a higher passband frequency
f.sub.ph for passing substantially the entire predetermined FDM
band, further having a lower stopband frequency f.sub.sl that is
less than said lower passband frequency and a higher stopband
frequency f.sub.sh that is greater than said higher passband
frequency, said passband including frequencies above a sample
frequency F.sub.s, at which the output of the filter is
sampled;
analog-to-digital conversion means for sampling the analog output
of said anti-alias filter to provide an aliased frequency, down
converted digital version of the received frequency division
multiplexed electromagnetic signals within the predetermined
frequency division multiplex band, wherein said analog to digital
conversion means has said sample frequency F.sub.s less than the
lowest frequency in the predetermined FDM band and is subject to
the following for at least one value of N which is a non-zero
positive integer:
whereby the output of the analog to digital conversion means
includes a non-distorted aliased image of the predetermined
frequency division multiplex band, said non-distorted aliased image
being located between 0 Hz and F.sub.s /2;
a gain controlled amplifier responsive to the output of said analog
to digital conversion means for controlling the gain of the analog
received RF signal provided to said analog to digital conversion
means;
digital complex frequency translation means for frequency
translating said aliased frequency down converted digital signal
providing a frequency translated digital signal having the center
of the selected channel at zero frequency; and
means for filtering said translated sampled digital received signal
to isolate said selected channel.
2. The receiver of claim 1 wherein said filtering means
comprises:
a first digital low pass filter responsive to said aliased
frequency down converted signal for providing a first filter
output; and
a first sample rate reducing circuit responsive to said first
filter output.
3. The receiver of claim 2 wherein said filtering means further
includes a second digital low pass filter responsive to said first
sample rate reducing circuit for providing a second filter
output.
4. The receiver of claim 1 wherein said filtering means
comprises:
first digital low pass filtering means responsive to said aliased
frequency down converted signal for providing a first filter
output; and
second digital low pass filtering means responsive to said first
filter output for providing a second filter output.
5. The receiver of claim 1 including a second frequency translating
means responsive to said filtering means for translating said
isolated selected channel to a predetermined intermediate
frequency.
6. The receiver of claim 5 wherein said second frequency
translating means comprises a digital complex mixer.
7. A frequency division multiplex receiver for isolating a selected
frequency division multiplex (FDM) channel in a predetermined FDM
band, comprising:
an RF amplifier;
an RF bandpass anti-alias filter having a passband between a lower
passband frequency f.sub.pl and a higher passband frequency
f.sub.ph for passing substantially the entire predetermined FDM
band, further having a lower stopband frequency f.sub.sl that is
less than said lower passband frequency and a higher stopband
frequency f.sub.sh that is greater than said higher passband
frequency, said passband including frequencies above a sample
frequency F.sub.s, at which the output of the filter is
sampled;
analog-to-digital conversion means for sampling the analog output
of said anti-alias filer to provide an aliased frequency down
converted digital version of the received frequency division
multiplexed electromagnetic signals within the predetermined
frequency division multiplex band, wherein said analog to digital
conversion means has said sample frequency F.sub.s less than the
lowest frequency in the predetermined FDM band and is subject to
the following for at least one value of N which is a non-zero
positive integer:
whereby the output of the analog to digital conversion means
includes a non-distorted aliased image of the predetermined
frequency division multiplex band, said non-distorted aliased image
being located between 0 Hz and F.sub.s 2;
a gain controlled amplifier responsive to the output of said analog
to digital conversion means for controlling the gain of the analog
received RF signal provided to said analog to digital conversion
means;
means responsive to said aliased frequency down converted signal
for generating a complex digital signal having in-phase and
quadrature components and having a spectral content that is
frequency translated relative to the frequency down converted
signal so that the center of the selected channel is at zero
frequency; and
means for filtering said digital signal to isolate said selected
channel.
8. The receiver of claim 7 wherein said filtering means
comprises:
a first digital low pass filter responsive to said aliased
frequency down converted signal for providing a first filter
output; and
a first sample rate reducing circuit responsive to said first
filter output.
9. The receiver of claim 8 wherein said filtering means further
includes a second digital low pass filter responsive to said first
sample rate reducing circuit for providing a second filter
output.
10. The receiver of claim 7 wherein said filtering means
comprises:
first digital low pass filtering means responsive to said aliased
frequency down converted signal for providing a first filter
output; and
second digital low pass filtering means responsive to said first
filter output for providing a second filter output.
11. The receiver of claim 7 including a second frequency
translating means responsive to said filtering means for
translating said isolated selected channel to a predetermined
intermediate frequency.
12. The receiver of claim 11 wherein said second frequency
translating means comprises a digital complex mixer.
13. A frequency division multiplex receiver for isolating a
selected frequency division multiplex (FDM) channel in a
predetermined FDM band, comprising:
an RF amplifier;
an RF bandpass anti-alias filter having a bandpass between a lower
passband frequency f.sub.pl and a higher passband frequency
f.sub.ph for passing substantially the entire predetermined FDM
band, further having a lower stopband frequency f.sub.sl that is
less than said lower passband frequency and a higher stopband
frequency f.sub.sh that is greater than said higher passband
frequency, said passband including frequencies above a sample
frequency F.sub.s at which the output of the filter is sampled;
analog-to-digital conversion means for sampling the analog output
of said anti-alias filter to provide an aliased frequency down
converted digital version of the received frequency division
multiplexed electromagnetic signals within the predetermined
frequency division multiplex band, wherein said analog to digital
conversion means has said sample frequency F.sub.s less than the
lowest frequency in the predetermined FDM band and is subject to
the following for at least one value of N which is a non-zero
positive integer:
whereby the output of the analog to digital conversion means
includes a non-distorted aliased image of the predetermined
frequency division multiplex band, said non-distorted aliased image
being located between 0 Hz and F.sub.s /2;
a gain controlled amplifier responsive to the output of said analog
to digital conversion means for controlling the gain of the analog
received RF signal provided to said analog to digital conversion
means;
a first digital complex mixer for frequency translating said
aliased frequency down converted signal pursuant to a selectable
local oscillator frequency to provide a frequency translated
sampled digital received signal having the center of the channel
represented by said local oscillator frequency located at zero
frequency;
digital low pass filtering means responsive to said frequency
translated sampled digital received signal for isolating said
selected channel; and
a second digital complex mixer for frequency translating said
isolated selected channel to a predetermined intermediate
frequency.
14. The receiver of claim 13 wherein said digital low pass
filtering means comprises:
a first digital low pass filter responsive to said aliased
frequency down converted signal for providing a first filter
output; and
a first sample rate reducing circuit responsive to said first
filter output.
15. The receiver of claim 14 wherein said digital low pass
filtering means further includes a second digital low pass filter
responsive to said first sample rate reducing circuit for providing
a second filter output.
16. The receiver of claim 13 wherein said filtering means
comprises:
first digital low pass filtering means responsive to said aliased
frequency down converted signal for providing a first filter
output; and
second digital low pass filtering means responsive to said first
filter output for providing a second filter output.
Description
BACKGROUND OF THE INVENTION
The disclosed invention relates generally to a radio frequency (RF)
or intermediate frequency (IF) receiver for frequency division
multiplexed signals, and more particularly is directed to a digital
RF/IF receiver for bandpass frequency division multiplexed (FDM)
signals such as frequency modulation (FM) radio broadcast
signals.
Frequency division multiplexed (FDM) communications utilizes
adjacent frequency bands or channels commonly characterized by
respective carrier frequencies, such frequency bands being in a
specified bandwidth which is typically wideband. A commonly known
example of wideband FDM communications is the amplitude modulation
(AM) radio broadcast band, which in the United States is fixed at
550 KHz to 1600 KHz with the channels spaced 10 KHz apart. Another
commonly known example of wideband FDM communications is the FM
radio broadcast band, which in the United States utilizes a 20 MHz
bandwidth, from 88 MHz to 108 MHz.
Typically, RF/IF receivers for FDM communications are mostly
analog, sometimes with some digital processing after the actual
tuner function (i.e., after the isolation of the selected
channel).
Important considerations with analog RF/IF receivers include the
necessity of precision circuit manufacturing techniques and the
attendant non-automated manual adjustments. Noise is a significant
undesired component and must always be carefully considered, from
design to assembly. Distortion must be considered throughout the
entire RF/IF receiver circuitry. Undesired mixer products are
present and may distort the channel of interest, and mixer local
oscillator feedthrough is a problem. Many of the analog components
are bulky and not amenable to integration, and moreover are subject
to drift over time and with temperature which must be considered
and reasonably compensated. Analog filters inherently have
non-linear phase characteristics.
SUMMARY OF THE INVENTION
It would therefore be an advantage to provide a digital RF/IF
receiver for bandpass frequency division multiplexed signals which
does not have the distortion, drift, and signal-to-noise ratio
limitations of analog IF receivers.
Another advantage would be to provide a digital RF/IF receiver for
bandpass frequency division multiplexed signals which is readily
manufactured with high yield mass production techniques.
The foregoing and other advantages are provided in a frequency
division multiplex receiver for isolating a frequency division
multiplex channel contained within a predetermined frequency
division multiplex (FDM) band. The receiver includes an RF
amplifier, and an RF bandpass anti-alias filter having a passband
between a lower passband frequency f.sub.pl and a higher passband
frequency f.sub.ph for passing substantially the entire
predetermined FDM band. The anti-alias filter further includes a
lower stopband frequency f.sub.sl that is less than the lower
passband frequency and a higher stopband frequency f.sub.sh that is
greater than the higher passband frequency. An analog-to-digital
converter samples the output of the anti-alias filter at a sample
frequency F.sub.s which is lower than the lower passband frequency
of the anti-alias filter, and is subject to the following for at
least one value of N which is a non-zero positive integer:
so as to produce a non-distorted aliased frequency down converted
digital version of the received frequency division multiplexed
signals in the predetermined frequency division multiplex band. The
frequency division multiplex receiver further includes a gain
controlled amplifier responsive to the output of the
analog-to-digital converter for controlling the gain of the analog
received RF signal provided to the analog-to-digital converter,
digital complex frequency translation means for frequency
translating the aliased frequency down converted digital signal to
provide a frequency translated digital signal having the center of
the selected channel at zero frequency, and filtering circuitry for
filtering the translated sampled digital received signal to isolate
the selected channel.
BRIEF DESCRIPTION OF THE DRAWING
The advantages and features of the disclosed invention will readily
be appreciated by persons skilled in the art from the following
detailed description when read in conjunction with the drawing
wherein:
FIG. 1 is a schematic block diagram of a digital IF receiver in
accordance with the invention for the particular example of an FM
band receiver.
FIG. 2 is a block diagram of one embodiment of the analog signal
processor of the digital IF receiver of FIG. 1.
FIG. 3 is a block diagram of a digital quadrature frequency
synthesized complex mixer which can be utilized as the complex
digital mixer of the digital IF receiver of FIG. 1.
FIG. 4 is a schematic illustration of the spectral characteristics
of an illustrative example of a sampled digital received FM
broadcast signal provided by the analog-to-digital converter of the
analog signal processor of FIG. 2.
FIG. 5 is a schematic illustrative example of the spectral
characteristics of the frequency translated digital received FM
broadcast signal provided by the complex mixer of the IF receiver
of FIG. 1.
FIG. 6 is a schematic illustration of the spectral characteristics
of a digital filter/re-sampler pair of the IF receiver of FIG.
1.
DETAILED DESCRIPTION
In the following detailed description and in the several figures of
the drawing, like elements are identified with like reference
numerals.
The invention relates to bandpass frequency division multiplexed
(FDM) communication systems which typically include adjacent
frequency bands or channels characterized by respective carrier
frequencies. For ease of reference, a particular channel being
selected or tuned for reception shall be referred to as the
selected channel or frequency, the latter referring to the carrier
frequency associated with the selected channel.
The FDM signals for a given communications system are typically
constrained to be within a specified bandwidth, which for ease of
reference is called herein the frequency division multiplexed
signal band or the FDM signal band.
Referring now to FIG. 1, shown therein is a digital RF/IF receiver
10 which by way of illustrative example will be described as an
RF/IF receiver for receiving FDM signals within the bandpass
frequency modulation (FM) radio broadcast band, which in the United
States occupies a 20 MHz bandwidth between 88 MHz and 108 MHz.
The digital RF/IF receiver 10 includes an analog signal processor
(ASP) 20 for receiving FDM signals within a predetermined bandpass
FDM band via an antenna 12, and provides a sampled digital received
signal R.sub.s which includes the FDM band of interest translated
to a lower frequency band. Example embodiments of the ASP 20 are
set forth in FIGS. 2 and 3.
Referring now to FIG. 2, the ASP 20A shown therein includes a radio
frequency (RF) amplifier 11 for receiving FDM signals within a
predetermined FDM signal band via the antenna 12. The output of the
RF amplifier 11 is provided to an RF bandpass anti-alias filter 13
which provides its filtered RF output to a gain controlled
amplifier (GCA) 14 which can be of known design. The output of the
GCA 14 is provided to a high speed precision analog-to-digital
(A/D) converter 15 which provides a sampled received signal
R.sub.s.
The GCA 14 is controlled by a periodically updated feedback digital
control word provided by a digital automatic gain control (DAGC)
processor 17 which is responsive to the output R.sub.s of the A/D
converter 15. The DAGC processor 17 can also be of known design and
includes peak detection circuitry and control word generating
circuitry. The control word is converted to a stable analog current
which is utilized to control the gain of the GCA 14.
The characteristics of the RF bandpass anti-alias filter 13 would
depend on the specific application and requirements, and preferably
should have very close to linear phase and should have minimum
loss. Generally, the RF anti-alias filter 13 has an appropriate
passband, defined at an appropriate attenuation level such as -3
dB, which extends from the lowest frequency to the highest
frequency of the FDM band of interest. Outside of the passband, the
location of the stopband edges, defined at an an appropriate
rejection level such as -100 dB, would depend on the A/D converter
sampling rate to the degree that the filter skirts (i.e., the
regions between a passband edge and the adjacent stopband edge)
from aliased spectral images do not encroach upon the passband of
the desired spectral image.
Pursuant to analyses known in the art, the sample rate of the A/D
converter 15 would depend on (a) the signal information bandwidth
and (b) aliased image location. Bandpass sampling allows for a
sample rate that is less than the frequency of the lower band edge
so long as the sample rate is at least twice the bandwidth of the
signal provided by the RF anti-alias filter 13. However, in order
to obtain a non-distorted aliased image located between 0 Hz and
one-half the sampling frequency F.sub.s with bandpass sampling, the
sample rate F.sub.s should be chosen to meet the following
requirements for one value of N which is a non-zero positive
integer:
where
f.sub.sl is the lower stopband edge of the anti-alias filter;
f.sub.pl is the lower passband edge of the anti-alias filter;
f.sub.ph is the higher passband edge of the anti-alias filter;
f.sub.sh is the higher stopband edge of the anti-alias filter.
The foregoing equations describe the necessary conditions for the
sample frequency F.sub.s with respect to the bandpass anti-alias
filter passband and stopband edges such that Nyquist's sampling
criterion is met, and all aliased image distortion is avoided. The
choice of N being odd or even in Equations 1 and 2 determines
whether the desired aliased image is spectrally not reversed (odd)
or is spectrally reversed (even). The boundary conditions defined
by Equations 1 and 2 provide a range of valid values of the
sampling frequency F.sub.s for a given N, but typically only a few
values of N, and often only one value, will yield a valid sampling
frequency F.sub.s. Equation 3 explicitly describes the Nyquist
criterion given finite bandpass filter skirts.
An example using the FM broadcast band with 8 MHz filter skirts
(e.g., -100 dB at the stopband edges) provides for the following
filter characteristics:
For N=1, Equation 1 becomes 84 MHz.gtoreq.F.sub.s .gtoreq.74.667
MHz, and Equation 3 becomes F.sub.s .gtoreq.48. Accordingly, 84 MHz
is an appropriate sample frequency which would produce a sampled
aliased image of the FDM band between 4 MHz and 24 MHz, as shown in
FIG. 4 which schematically depicts the spectral content of the
sampled received signal output R.sub.s of the A/D converter 15 for
the FM broadcast implementation having the anti-alias filter
characteristics described above and a bandpass sample frequency of
84 MHz. As is well known, the spectral content of a single channel
analog filtered and sampled signal includes a negative image and
aliased images due to sampling. In FIG. 4, the positive and
negative mirror images which lie within the original FDM band are
shaded.
As another example, for an anti-alias filter having the
above-described characteristics and for N=2, Equation 2 becomes 56
MHz.gtoreq.F.sub.s .gtoreq.56 MHz, and Equation 3s become F.sub.s
.gtoreq.48. Thus, 56 MHz is the only appropriate sampling frequency
for N=2, which would produce a non-distorted reversed aliased image
that is between 24 MHz and 4 MHz.
It should be noted that if N is selected to be 3, the conditions of
Equation 1 are not satisfied, and there is no appropriate sampling
frequency for N=3.
From the foregoing it should be appreciated that sampling at a
lower frequency than the lowest frequency in the passband of
interest results in a frequency down conversion from the RF or IF
frequency to sampled baseband, where sampled baseband is from 0 Hz
to one-half the sampling frequency. Since frequency down conversion
is achieved with bandpass sampling, analog frequency down
conversion is avoided.
For ease of understanding of the circuitry downstream of the analog
signal processor 20, the FM broadcast illustrative example will be
described relative to a sample rate of 84 MHz at the output of the
ASP 20.
The sampled digital received signal R.sub.s of the ASP 20 is
provided to a digital complex mixer 19 which by way of example is
shown in FIG. 3 as a digital quadrature frequency synthesis mixer.
It should be readily appreciated that the term "complex" refers to
the output of the mixer 19 which includes in-phase and quadrature
components (I and Q) that can be mathematically represented with
"complex numbers," as is well known in the art. In complex number
representations, the in-phase and quadrature components are
commonly called "real" and "imaginary" components.
Complex mixing is utilized since this allows the entire spectrum to
be shifted in one direction, as distinguished from "real" mixing
(i.e., where only one multiplication is utilized) which can result
in distortion producing overlapping images. As is well known, real
mixing produces four images of the original positive and negative
spectral images. As to each of the original images, the output of a
"real" mixer includes two images displaced positively and
negatively relative to the location of the original image, and
inappropriate choice of the local oscillator frequency could result
in distortion due to overlapping images.
The digital complex mixer of FIG. 3 includes a digital quadrature
frequency synthesizer 111 which receives an input control signal
indicative of the selected channel to be tuned. The digital
frequency synthesizer 111 can be of known design and provides
sampled digital sine and cosine outputs having the same frequency
as the carrier frequency of the selected channel to be tuned. In
traditional terminology, the outputs of the digital quadrature
frequency synthesizer 111 can be considered the local oscillator
(LO) quadrature outputs.
The cosine output of the digital quadrature frequency synthesizer
111 is provided as one input to a first multiplier 119, while the
sine output of the digital quadrature frequency synthesizer 111 is
provided as one input to a second multiplier 121. The sampled RF
signal R.sub.s is coupled as further inputs to both the first
multiplier 119 and the second multiplier 121.
The outputs of the multipliers 119, 121 are respectively the
in-phase and quadrature components (I and Q) of a complex signal
which includes the desired sampled aliased FDM band image (which
was between 4 MHz and 24 MHz in the illustrative FM broadcast
example) translated in frequency with the the selected FDM channel
centered at zero frequency (DC). This frequency translation is
determined by the frequency of the output of the digital quadrature
synthesizer 111 which in turn is controlled by its input control
signal. The spectral characteristics of the complex output of the
digital complex mixer 19 for the FM broadcast example is
schematically shown in FIG. 5.
Since the output of the complex mixer 19 includes components in
addition to the selected channel (e.g., shifted aliased images and
unselected channels), low pass filtering is required to isolate the
selected FDM channel that is centered at DC. Such filtering
includes respective non-complex filtering for the in-phase and
quadrature components, with the filter coefficients having only
"real" components; i.e., each filter coefficient only has one
component and does not have an "imaginary" component.
The low pass filtering of the output of the complex mixer 19 can be
provided by a single digital filter having appropriately sharp
cutoff and linear phase characteristics. Preferably, however,
cascaded low pass filter and re-sampler pairs are utilized to
provide for more efficient filter operation and less complicated
filter structures. With cascaded filter/re-sampler pairs, the
passband edge of each filter is the same as the passband edge of
the desired channel that is centered at DC. The stopband edge of a
given filter is determined by the re-sample rate to be applied to
the filter output as well as the passband edge. The amount of
stopband suppression for each filter is determined by the allowable
alias criterion for the overall system. For background information
on cascaded filter/re-sampler circuits, reference is made to
Chapter 5 of Multirate Digital Signal Processing, Crochiere and
Rabiner, Prentice-Hall, Inc., Englewood Cliffs, N.J. 07632, 1983,
and particularly to pages 193-250.
For the FM broadcast example, appropriate composite low pass
filtering provided by multi-stage filtering can include a passband
from DC to 75 KHz and approximately 100 dB stopband suppression
beginning at about 125 KHz.
Continuing with our illustrative FM broadcast example, the complex
output of the digital complex mixer 19 is provided to a first
digital low pass filter 21 which can comprise, for example, a
finite impulse response (FIR) filter or an infinite impulse
response (IIR) filter of known configuration. The output of the
first digital low pass filter 21 is provided to a first re-sampler
circuit 23 which reduces the sample rate. In the FM broadcast
example, the illustrative sample rate of 84 MHz is reduced by a
factor of 1/4 to 21 MHz.
The output of the re-sampler 23 is provided to a second digital low
pass filter 25 which provide further low pass filtering. The output
of the digital filter 25 is provided to a second re-sampler 27
further reduces the sample rate. In the FM broadcast example, the
sample rate of 21 MHz is reduced by a factor of 1/4 to 5.25
MHz.
The output of the re-sampler 27 is provided to a third digital low
pass filter 29 which provides further low pass filtering. The
output of the filter 29 provided to a third re-sampler 31 to reduce
the sampling rate. In the FM broadcast example, the sample rate of
5.25 MHz is reduced by a factor of 1/4 to 1.3125 MHz.
The output of the re-sampler 31 is provided to a fourth digital low
pass filter 33 which provides still further low pass filtering. The
output of the filter 33 is coupled to a fourth re-sampler 35 which
further reduces the sampling rate. For the FM broadcast example,
the sample rate of 1.315 MHz is reduced by a factor of 1/2 to
0.65625 MHz or 656.25 KHz.
FIG. 6 schematically depicts the spectral characteristics of one of
the above-described filter/re-sampler pairs, generally illustrating
the foldback of filter skirts around the half sample frequency via
aliasing as a result of re-sampling. Such foldback can be a source
of distortion in the baseband passband region if the filter
stopbands are not appropriately suppressed.
The output of the fourth re-sampler 35 is provided to a final
digital low pass filter 37. The output of the digital low pass
filter 37 includes the selected FDM channel isolated and centered
at DC.
Depending on the chosen demodulator that processes the output of
the digital IF receiver 10, the output of the digital low pass
filter 37 may be provided to a digital complex mixer 39 which
translates the selected FDM channel to be centered at a
predetermined IF frequency. The digital complex mixer 39 can be
similar to the digital complex mixer 19 discussed above, except
that the complex mixer 39 utilizes a fixed LO frequency and has
complex data inputs. Essentially, the complex mixer 19 multiplies
the complex output of the low pass filter 37 by a complex local
oscillator frequency. Each sample output of the low pass filter 37
can be represented by the complex number (A+jB), and the local
oscillator phase at a given sample time can be represented by the
complex number (cos(z)+j sin (z)), where j represents the square
root of -1. The complex multiplication achieved by the complex
mixer is as follows: ##EQU1## where (A cos (z)-B sin (z)) is the
in-phase or real component and (B cos (z)+A sin (z)) is the
quadrature or imaginary component at the sample time. Of course,
the complex mixer 39 can be implemented with techniques known in
the art that efficiently reduce or eliminate actual
multiplications.
The in-phase component of the output of the digital complex mixer
39 represents a very low distortion version of the selected FDM
channel centered at an IF frequency which is symmetrical about DC
in the frequency domain. Specifically in the illustrative FM
broadcast example, the in-phase component of the output of the
complex mixer 39 represents the selected frequency division
multiplexed channel which is ready to be digitally de-modulated,
and decoded, for example for FM stereo.
Although the foregoing digital IF receiver has been discussed to
some extent in the context of receiving FM broadcast signals, the
invention contemplates frequency division multiplexed
communications in general. For other applications, the sample
rates, filter characteristics, and other parameters would obviously
have to be determined. As appreciated by persons skilled in the
art, such determinations would be based upon filtering parameters
for known analog systems, desired optimization, signal-to-noise
ratio requirements, and other factors individual to each
application.
The disclosed digital IF receiver provides advantages including the
virtual elimination of mixer local oscillator feedthrough, local
oscillator print-through (alteration of the local oscillator
frequency due to intermodulation distortion), filter phase
non-linearity, and IF blanketing (IF difference mixing caused by
mixer products which comprise two different FDM channels). The
performance capability can be made arbitrarily high in quality
depending upon the linearity and resolution of the RF amplifier and
the analog-to-digital converter, the complexity of the digital
filters, and upon the digital wordsize utilized in the receiver.
The processing is independent of information content and
modulation. Signal-to-noise ratio is better due to sharp
linear-phase digital filtering and re-sampling. Spurs caused by IF
intermodulation distortion and errant mixer products are virtually
eliminated.
The digital RF/IF receiver of the invention is readily amenable to
integration, can be made on a few VLSI chips, and has excellent
manufacturability. Precision circuit techniques are not required
beyond the amplifiers, the analog filters, an analog mixer if
utilized, and the analog-to-digital converter, which allows the
balance of the digital IF receiver to be reliably and consistently
produced. Digital filters can readily be made to have superior
phase linearity in comparison to analog filters. Moreover, the
digital RF/IF receiver utilizes a sampling frequency that is
selected with reference to analog anti-alias filtering
characteristics which advantageously avoids analog mixing to
produce a non-distorted frequency down converted aliased image.
Although the foregoing has been a description and illustration of
specific embodiments of the invention, various modifications and
changes thereto can be made by persons skilled in the art without
departing from the scope and spirit of the invention as defined by
the following claims.
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